1. Field of the Invention
The present invention relates to a motor vehicle steering system such as an electric power steering system.
2. Description of Related Art
In electric power steering systems, for example, three-phase blushless motors are employed as driving sources. The three-phase blushless motor is subjected to feedback control on the basis of the operation of a steering wheel, so that a steering assist force responsive to the operation of the steering wheel is produced from the three-phase blushless motor.
Specifically, the electric power steering system comprises a three-phase inverter circuit for driving the three-phase blushless motor, a microcomputer for controlling ON/OFF of switching elements provided in the three-phase inverter circuit, and current sensors for detecting the respective values of currents supplied to phases of the three-phase blushless motor from the three-phase inverter circuit. The current sensor is provided for each of the phases. A detection signal of the current sensor is inputted to the microcomputer. The microcomputer determines an each-phase target current value responsive to the operation of the steering wheel. Each of the switching elements provided in the three-phase inverter circuit is turned on/off by a duty corresponding to a deviation between the each-phase target current value and the each-phase current value detected by each of the current sensors. Consequently, currents having the each-phase target current values are respectively supplied to the phases of the three-phase blushless motor, so that a steering assist force responsive to the operation of the steering wheel is generated from the three-phase blushless motor.
However, non-uniformity occurs in the rotation of the three-phase blushless motor due to variations in sensitivity (properties) among the current sensors, which may cause torque ripple to be generated in the three-phase blushless motor. The torque ripple in the three-phase blushless motor causes the steering wheel to be vibrated, which degrades a steering feeling.
Since the current sensor is provided for each of the phases of the three-phase blushless motor, the cost is high.